The present invention relates generally to photonic devices. more specifically, the invention de-couples the traditional trade off between device resistance as a function of contact size and precision of the aperture. It is to be appreciated that the present invention is amenable to many applications such as, Vertical Cavity Surface Emitting Lasers (VCSEL""s), resonant cavity LED""s, photodetectors, edge emitting lasers, and LED""s.
There are many semiconductor devices that require within their structure an electrically conductive region bounded by an electrically insulating region. Examples of such devices include vertical cavity surface-emitting lasers (VCSELs), edge-emitting lasers, light-emitting diodes (LEDs) and photodetectors. In many of these devices, the electrically insulating region should preferably have a lower refractive index than the conductive region. Recently, there has been an increased interest in forming the electrically insulating region by laterally oxidizing at least one semiconductor layer. The oxidation process selectively converts the conducting semiconductor layer into a lower refractive index insulator.
A known technique to fabricate many photonic devices can be illustrated by a VCSEL formed by a lateral oxidation process, schematically illustrated in FIGS. 1 and 2. Under this approach, a laser structure comprising a plurality of layers is formed upon substrate 10. These layers include an active layer 12 and an AlGaAs layer 14 with a high aluminum content. The AlGaAs layer 14 is placed either above or below the active layer of a laser structure. Then, the layered structure is masked and selectively etched to form a mesa structure 22 as illustrated in FIG. 2. As a result of the etching, the AlGaAs layer 14 with a high aluminum content adjacent to the active layer 12 is exposed at the edges of the mesa structure 22. To form the lasing emissive region or xe2x80x9caperturexe2x80x9d, this AlGaAs layer is oxidized laterally from the edges towards the center of the mesa structure as represented by arrows A. Other layers in the structure remain essentially unoxidized since their aluminum content is lower. Consequently, their oxidation rates are also substantially lower. Therefore, only the AlGaAs layer with high aluminum content is being oxidized. The oxidized portions of the high aluminum content layer become electrically non-conductive as a result of the oxidation process. The remaining unoxidized region, which is conductive, in the AlGaAs layer forms the so-called xe2x80x9caperturexe2x80x9d, a region which determines the current path in the laser structure, and thereby determines the region of laser emission. A VCSEL formed by such a technique is discussed in xe2x80x9cSelectively Oxidized Vertical Cavity Surface Emitting Lasers With 50% Power Conversion Efficiency,xe2x80x9d Electronics Letters, vol. 31, pp.208-209 (1995).
The most common lateral oxidation approach has several disadvantages, such as large mesa, large oxidation region, and poor control of the aperture size. A key disadvantage of this approach is the difficulty in controlling the amount of oxidation. Generally, the desired device aperture is on the order of one to ten microns (xcexcm), which means that several tens of microns of lateral oxidation will typically be required in order to fabricate the device when oxidizing in from the sides of the much larger mesa, which must typically be 50 to 100 microns in diameter. Since the size of the resulting aperture is small relative to the extent of the lateral oxidation regions, the devices formed generally have severe variations in aperture size as a result of non-uniform oxidation rates from wafer to wafer and across a particular wafer. The oxidation rate of AlGaAs depends strongly on its aluminum composition. Any composition non-uniformity will be reflected by changes in the oxidation rate, which in turn creates uncertainty in the amount of oxidation. The process is also relatively temperature-sensitive. As the oxidation rate varies, it is difficult to ascertain the extent to which a laser structure will be oxidized, thereby decreasing reproducibility in device performance. In short, such a process often creates various manufacturability and yield problems.
Another disadvantage of a photonic device formed by a traditional lateral oxidation approach is that they often suffer from poor mechanical or structural integrity. It is well-known that the upward pressure applied during a packaging process may cause delamination of the entire mesa since the bonding of the oxide layer to the unoxidized GaAs or AlGaAs is generally weak.
Another known technique to fabricate VCSEL""s, especially in highly compact arrays is known, and is described for example in U.S. Pat. No. 5,978,408 issued on Nov. 2, 1999 to Thornton. In such highly compact arrays, a number of laser apertures are packed closely together in various geometric shapes. Due to the closeness of the apertures however, relatively small metal contacts are used to bias the laser. Use of these smaller contacts has undesirably raised the resistance of these semi-conductor lasers, thus increasing the voltages required to drive them.
Accordingly, a need exists to separate the interdependence of the size of the electrical contact area and the accuracy with which a desired aperture size can be obtained.
The present invention contemplates a new and improved method and apparatus for forming photonic devices which overcome the above-referenced problems and others.
The present invention provides a photonic structure, such as lasers, light emitting diodes or photo-detectors, having well-defined and well-controlled oxidized regions, which can be used to define the aperture of the structure. These oxidized regions are formed by the use of a multiplicity of channels arranged in a pattern in the structure. During the oxidation process, an AlGaAs layer with high aluminum content embedded in the semiconductor structure is oxidized radially outwards from each of these channels until overlap in the oxidized regions defines a central conductive aperture. The device aperture is the unoxidized region bounded by these oxidized regions centered about the channels. The AlGaAs layer with high aluminum content for forming the oxidized regions and the aperture is often referred to as an xe2x80x9coxidation layer.xe2x80x9d
The present invention further provides a semi-conductor photonic structure having a large electrical contact. Larger contacts desirably tend to reduce the resistance of a device. The combination of the large contact size and the precisely placed and oxidized apertures effectively de-couple the usual tradeoff between device resistance and obtainable aperture precision.
In accordance with one aspect of the present invention, a photonic device includes a substrate, and a plurality of semi-conductor layers formed on the substrate. The semi-conductor layers include an active layer and a current controlling region near the active layer. The current controlling region includes an electrically insulating section and an electrically conductive section. The current controlling region is penetrated by a plurality of channels surrounding or bounding the electrically conductive section within their perimeter. An electrical contact is also provided on the laser such that at least part of the electrical contact lies outside the perimeter of the channels.
In accordance with another aspect of the present invention, the electrical contact includes an area greater than an area within the perimeter of the channels.
In accordance with another aspect of the present invention, the electrical contact includes an area at least two times larger than the area of the electrically conductive section.
In accordance with another aspect of the present invention, the current controlling region includes an oxidation layer. The electrically insulating section includes an oxidized portion of the oxidation layer. The electrically conductive section includes an unoxidized portion of the oxidation layer.
In accordance with another aspect of the present invention, the current controlling region further includes an electrically insulating ion implanted layer extending from a perimeter of the device or field implant region to at least an area adjacent to the oxidized portion of the oxidation layer.
In accordance with another embodiment of the present invention, a method of forming a photonic device includes forming a plurality of spaced channels in a surface of a semi-conductor layer bounding a desired aperture. The method further includes affixing an electrical contact to the semi-conductor layer, where the electrical contact has an area at least two times greater than an area of the desired aperture.
In accordance with another aspect of the present invention, the method further includes altering selected portions of at least one layer from a generally conductive state to a generally non-conductive state.
In accordance with another aspect of the present invention, the channels permit communication between a surface of the semi-conductor layer and a buried oxide layer. The altering step includes oxidizing a selected portion of the oxide layer.
In accordance with another aspect of the present invention, the altering step further includes implanting at least one layer adjoining the oxidized portion of the oxide layer with ions at a selected energy level.
In accordance with another aspect of the present invention, the electrical contact includes an interruption substantially corresponding to the desired aperture. When attached, the interruption in the contact aligns with the desired aperture.
An advantage of the present invention is that the dimension of the oxidized regions which define the lasing aperture is comparable to the dimension of the lasing aperture itself. Generally, the oxidation rates of AlGaAs depend upon material composition and process parameters, which exhibit significant non-uniformity. These non-uniformities have decreasing impact as the ratio of the size of the oxidized regions to the size of the final laser aperture is reduced. In other words, since the amount of oxidation required to form the lasing aperture is dramatically reduced, the aperture size is therefore much less sensitive to material or process variations. Hence, well-defined and well-controlled apertures can be achieved.
Another advantage of the present invention is that since the present invention does not need small total device size to improve the precision of the apertures, the size of the electrical contact can be increased to reduce the overall device resistance.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the invention, its preferred embodiments, the accompanying drawings, and the appended claims.